If you are trying to add structured text into RichTextBox, you are going to be using the Rtf property which accepts a string that has text formatting in it. As you haven't actually shown any code, we can't offer much more support other than saying that you are going to have to deal with the RTF yourself. So, if I wanted to have the following string:
This is extremely bold text with some italics in it and a strikethrough
this is going to end up looking like this as the RTF string.
{\rtf1\ansi\ansicpg1252\deff0\nouicompat\deflang2057{\fonttbl{\f0\fnil\fcharset0 Calibri;}}
{\*\generator Riched20 10.0.22621}\viewkind4\uc1
\pard\sa200\sl276\slmult1\f0\fs22\lang9 This is extremely \b bold\b0 text with some \i italics\i0 in it and a \strike strikethrough\strike0 .\par
}